Record carrier on which information is stored in an optically readable structure with dither focussing signals also being stored

ABSTRACT

A record carrier is described on which information is stored in an optically readable structure of trackwise arranged areas and intermediate areas. By giving at least one of the surfaces of the record carrier body excursions which are periodically variable in the track direction, it is possible in a simple manner to obtain an indication of the focussing of the read beam on the information structure during reading of the record carrier.

The invention relates to a record carrier on which information is storedin an optically readable structure of areas and intermediate areasarranged along tracks, which areas have a different influence on a readbeam of radiation than the intermediate areas. The invention alsorelates to an apparatus for reading such a record carrier.

In this specification tracks is to be understood to mean those trackportions which, viewed in the lateral direction of the track portions,are adjacent to each other. For a round disk-shaped record carrier atrack is a track portion which extends along one revolution on therecord carrier. A spiral track is the total of merging quasi-concentrictracks on a round disk-shaped record carrier. A record carrier consistsof a record carrier body on which an information structure is disposedand, as the case may be, a protective layer on said informationstructure.

It has been proposed, for example in the previous U.S. PatentApplication Ser. No. 229,285, filed Feb. 25, 1972, abandoned in favor ofContinuation Applicaton Ser. No. 396,399, filed Sept. 12, 1973, whichhas been laid open for public inspection, to store a color televisionprogram in a record carrier as described above. For example, a rounddisk-shaped record carrier may contain a 30 -minute in a multitude oftracks, the outer track having a radius of approximately 15 cm and theinner track a radius of approximately 8 cm. The period, in the radialdirection, of the tracks is then approximately 2 μ m.

The record carrier can be read optically by focussing a read beam ofradiation onto the optical structure, and moving the radiation spot thusformed and an information track relative to each other. The read beam ofradiation is then modulated in accordance with the sequence of areas andintermediate areas in a track. The modulated read beam is converted by aradiation-sensitive detector into an electrical signal from which forexample picture and/or sound may be derived.

When reading the record carrier minute details, in the above exampledetails of the order of magnitude of microns, must be detected. Forthis, use is made of an objective system with a high numerical apertureand a small depth of focus. Said objective system must ensure than theread beam is always sharply focussed onto the information structure.Owing to for example errors in the bearing of the record carrier orwarping of the record carrier, or as a result of vibrations in thereading apparatus, it may happen that the track to be read moves axiallyrelative to the objective system. The detector then does not onlyreceive radiation from the track portion to be read, but also radiationfrom the vicinity of said portion. As a result the modulation depth ofthe signal supplied by the detector decreases, while moreover cross-talkmay occur because not only one track is read but adjacent tracks arealso illuminated. Therefore, is is of importance to be able to detect adeviation between the plane of focussing and the plane of theinformation structure to be read, so as to enable the focussing to becorrected.

It is an object of the present invention during reading of a recordcarrier of the type mentioned in the preamble to produce, in addition toan information signal, a focussing correction signal, the last-mentionedsignal being obtained with a minimum of optical elements. In order toachieve this, a record carrier according to the invention ischaracterized in that at least one surface of a record carrier bodyexhibits periodic excursions, which excursions vary in the direction inwhich the tracks are read, and that the period of said excursions issubstantially greater than the average period of the areas in thetracks.

The periodic excursions cause the read beam to be periodically focussedand defocussed on the plane of the track portion to be read. As aresult, the signal supplied by the detector will be modulatedperiodically. From the variation of said modulation the magnitude of adeviation between the actual and the desired position of the plane ofthe track portion to be read can be derived.

The period of the excursions must be so much greater than the averageperiod of the areas that in the detected signal the modulation as aresult of the information may be readily distinguished, as regardsfrequency, from the modulation as a result of the periodical excursions.For a record carrier provided with a television program the period ofthe excursions is some hundreds of times greater than the average periodof the areas in a track. The amplitude of the excursions will have to beselected so that the plane of the information structure is always withinthe depth of focus of the objective system. Said amplitude may be of theorder of microns.

In principle, a record carrier according to the invention can be readwith an apparatus employing only one beam of radiation and one detector.In an electric circuit, which is connected to the detector, ahigh-frequency component of the detector signal is separated from alow-frequency component. Said components are processed to an informationsignal and a control signal for focussing correction respectively. Asfor obtaining the control signal no additional optical elements arerequired, alignment problems which would occur during assembly of theread apparatus if the said additional elements were employed, areavoided. Moreover, no extra provisions need be taken in the readapparatus to compensate for vibrations which might occur between theadditional optical elements during reading.

It is to be noted that in U.S. Pat. No. 3,673,412 it has been proposedwhen reading a record carrier provided with an optical informationstructure to derive both the information and the control signal forfocussing of the read beam from the signal supplied by a singledetector. In the known apparatus an oscillatory motion in a directionperpendicular to the record carrier is impressed on the radiation sourcewhich supplies the read beam. As a result, the read beam is periodicallyfocussed and defocussed on the information structure. Since mechanicalelements must be reciprocated the attainable frequency of oscillation isrelatively low, too low to be used for reading a record carrier with ahigh information density, such as a record carrier with a televisionprogram. Furthermore, the known read apparatus is fairly intricate owingto the mechanical oscillation means.

The invention will now be explained by describing its use in a rounddisk-shaped record carrier. For this, reference is made to the drawing,in which:

FIG. 1 shows a part of a known record carrier in top view,

FIG. 2 schematically shows an apparatus for reading a record carrier,

FIG. 3 in tangential cross-section shows a small part of a rounddisk-shaped record carrier according to the invention,

FIG. 4 shows a bottom view of such a record carrier,

FIGS. 5a, 5b and 5c illustrate the principle underlying the invention,

FIG. 6 shows the time response of the signal derived from the recordcarrier according to the invention for various settings of the readobjective,

FIG. 7 shows some embodiments of a record carrier according to theinvention.

On the round disk-shaped record carrier 1 shown in FIG. 1 a multitude ofconcentric or quasiconcentric tracks 2 are provided, of which only a feware shown. Between the information tracks structureless lands 3 aredisposed. The tracks 2 comprise a multiplicity of areas g alternatingwith intermediate areas t. The lengths of the areas and the intermediateareas represent the stored information. When reading a track a read beamis modulated, so that said beam exhibits time variations in accordancewith the sequence of areas and intermediate areas in a track.

The areas in a track can be distinguished from the intermediate areasand the lands between the tracks in various manner. For example, thetransmission or the reflection coefficient of the areas may differ fromthose of the intermediate areas, so that a read beam is amplitudemodulated. Alternatively, a read beam may be phase-modulated by theareas and the intermediate areas. This can be achieved by disposing theareas and the intermediate areas at different levels in the recordcarrier. Preferably, such a phase structure consists of a number of pitswhich are pressed in a reflecting record carrier at a depth of λ/4, λbeing the wave length of the radiation used for reading. Such astructure may for example be read with a radiation spot, or read spot, V(see FIG. 1) whose diameter is greater than the track width, but smallerthan the sum of the widths of one track and two lands.

FIG. 2 shows an apparatus for reading a record carrier. The read beamemitted by a source 6 is reflected to an objective lens 8 by asemi-transparent mirror 7. Said objective lens forms a read spot V onthe optical structure which contains the information, which isrepresented by the tracks 2. The optical structure, by way of example,is shown at the top surface of the record carrier. Upon reflection atthe information structure the read beam passes through the objectivelens 8 for a second time and said beam is then concentrated onto aradiation-sensitive detector 9 via the semi-transparent mirror 7. Theoutput of the detector 9 is connected to an electronic circuit 10, inwhich the detector signal is processed into an information signal Si,which for example is fed to a conventional color television receivingapparatus 11. If the radiation spot is projected onto an area g on therecord carrier, the radiation is diffracted, so that a substantialportion of the reflected radiation falls outside the entrance pupil ofthe objective lens 8, and thus outside the radiation-sensitive area ofthe detector 9. When the radiation spot is projected outside an area gon the record carrier, most part of the reflected radiation isconcentrated onto to the detector by the objective lens. The recordcarrier can be rotated with the aid of a shaft 5 which extends through acentral opening 4 in the record carrier. By moving the read spot over atrack the detector signal is modulated in accordance with the sequenceof areas and intermediate areas in the track.

During reading deviations between the actual and the desired position ofthe plane of the track to be read may occur. Said deviations may becaused by out-of-flatness of the record carrier or, in the case of around foil-like record carrier, by undulations which occur duringrotation of the record carrier. Said deviations may result in a decreaseof the modulation depth of the detector signal, while moreovercross-talk may occur. As a result, it is no longer possible to read therecord carrier correctly. Therefore, it is essential that the deviationbetween the actual and the desired position of the plane of the track tobe read can be detected.

According to the invention said deviation may be detected without theuse of additional optical elements. For this purpose, a surface of therecord carrier body exhibits periodical excursions, the direction of theperiod of the excursions being the direction in which the tracks areread.

FIG. 3 shows an embodiment of a record carrier according to theinvention in tangential cross-section. Only a small part of one track isshown. The record carrier is assumed to be a thin, foil-like recordcarrier. The areas g, which represent the information, and theintermediate areas t are disposed at the upper surface of the recordcarrier. The lower surface of the record carrier body exhibits anundulating or bumpy surface. FIG. 4 shows a bottom view of the entirerecord carrier. The radially extending lines 1 connect points of equalheight at the surface of the record carrier.

It is to be noted that in FIG. 3 the lengths of the areas and of theintermediate areas have been exaggerated relative to the period p of theexcursions. In reality the period p is of the order of 100 to 1000 timesas great as the average period of the areas g. When in FIG. 4 all topsof the surface were interconnected in a radial direction, the number oflines would have to be of the order of 100 to 1000.

The effect of the undulating shape, in the record carrier body surfaceis clarified by means of FIGS. 5a, 5b and 5c. Said Figures showcross-sections of each time other parts of a record carrier according tothe invention. The thickness of the part shown in FIG. 5b is exactlysuch that the read beam is focussed onto the top surface of the recordcarrier, where the information structure is assumed to be located, bythe objective lens 8. A read spot V of minimal size is obtained at thetop surface of the record carrier. The thickness d' of the part of therecord carrier body shown in FIG. 5a is smaller than d. The plane offocussing of the objective lens 8 is then located inside the recordcarrier body. The read spot V' at the top surface of the record carrierbody is larger than the read spot V. In FIG. 5c the thickness d" of therecord carrier body is greater than d. The plane of focussing of theobjective lens is now located above the record carrier and the read spotV" is larger than the read spot V.

Owing to the periodic excursions of the surface of the record carrierthe read beam is periodically focussed and defocussed on the plane ofthe information structure. As a result, the modulation depth of thedetector signal will change periodically, at low frequency, as is shownin FIG. 6. Said Figure shows how the intensity I of the detector signalvaries as a function of time, when the read spot moves over a track inthe direction of reading. It is assumed that the read beam is focussedat the level m, being the average level of the undulating surface. Thepulses, whose pulse widths and pulse spacing represent the information,are amplitude modulated by the undulating shape of the surface. From thedetector signal a low-frequency component can be derived with a responseas represented by the curve S in FIG. 6.

If the read beam is not fucussed at the level m, but at a level m', thesaid low-frequency component will have a response as that of the curveS' in FIG. 6. Whereas S only has the frequency corresponding to twicethe spatial frequency of the periodic excursions, S' also exhibits thefundamental frequency corresponding to the spatial frequency of theperodic excursions. The instants at which the maxima in the curve S' areattained are shifted by a time interval t₁ relative to the instants atwhich the maxima in the curve S are attained. When the read beam isfocussed on the level m", the low-frequency component will have a shapelike that of the curve S" . Said component also has the fundamentalfrequency, but now in phase-opposition relative to S' , so that the timeintervals at which the maxima are attained are shifted by a timeinterval t₂ relative to the instants at which the maxima in the curve Sare attained. However, the shift is opposed to that which occurs in thecase of the curve S' . The magnitude of a defocussing, if any, can bederived from the amplitude of the low-frequency component in thedetector signal.

In an electronic circuit (10 in FIG. 1) which is connected to thedetector a high-frequency and a low-frequency component can be derivedfrom the detector signal, and the low-frequency component can beprocessed into a control signal S_(c) with which the focussing can becorrected. The signal S_(c) for example allows the focal length of theobjective lens 8 to be changed. The objective lens may alternatively besuspended in a loudspeaker coil, and the signal S_(c) may then beapplied to said coil, so that the objective lens can be moved.

As is evident from FIG. 6, the direction of a defocussing may be derivedfrom the low-frequency component of the detector signal. The phase ofsaid component must then be compared with a reference phase. For thispurpose, the record carrier may for example be provided with additionalphase reference marks, which marks for example mark the points ofmaximum excursion of the undulating surface per revolution. Said marksmay be detected by a separate read head. The marks may, for example, bemechanically, magnetically or optically detectable marks. In the eventof a round disk-shaped record carrier the reference marks may beprovided in a separate track at the inner or outer circumference of therecord carrier.

When during manufacture of the record carrier it is possible toestablish a fixed relationship between the variation of the periodicexcursions of the undulating surface and the location of the informationin the tracks, the reference mark may be provided in the informationtracks, for example at the locations which correspond to the linesynchronizing pulses if a television program is stored on the recordcarrier.

FIG. 7 schematically shows some embodiments of a record carrieraccording to the invention. The information structure is represented bya dash line. The unmodulated read beam is represented by a single arrowand the modulated beam by a double arrow. In the cases (a), (b) and (c)the information structure is a transmission structure, whereas in theother cases a reflection structure is provided. The amplitude of theperiodic defocussing, as caused by the periodic excursions of theundulating surface is:

in cases a and b proportional to ##EQU1## in cases b and c proportionalto ##EQU2## and in cases c and f proportional to b, where

n₁ is the refractive index of the material of the record carrier,

n₂ is the refractive index of the surrounding medium, and

b represents the amplitude of the periodic excursions.

For most record carrier materials the refractive index is approximately1.5 . If the record carrier is located in air, the amplitude of theperiodic defocussing is:

in cases a and d proportional to ##EQU3## in cases b and e proportionalto ##EQU4## in cases c and f proportional to b.

To obtain the same effect, the amplitude of the excursions in cases aand d must be three times as high as in the cases c and f . Theembodiments a and d , where the information is not stored at theundulating surface, may be selected only in the case of thin recordcarriers (foils). The diameter of the read beam must be small relativeto the period of the excursions. If the read beam is focussed onto aninformation structure which is not situated at the undulating surface(cases a and d ), the diameter of said beam at the corrugated surfacewill be greater in the case of a thick record carrier.

With respect to the amplitude of the excursions of the undulatingsurface it is to be noted that the variations should remain within thedepth of focus of the objective lens. The amplitude is determined by thepermissible variations in modulation depth of the detector signal whichmay be introduced in the read beam by the undulation.

A record carrier with a reflecting information structure is to bepreferred over a record carrier with a radiation-transmittinginformation structure, because less optical elements are required in theread apparatus. Some of the optical elements are traversed twice, sothat vibrations between said elements have virtually no effect on thedetector signal. Preferably use is made of a thick record carrier inaccordance with e in FIG. 7. The record carrier body then serves as aprotective layer for the information, and ensures that scratches, dustparticles and the like have substantially no influence on the readprocess. On the information structure of the record carrier according toFIG. 7 an additional protective layer may be provided.

It will be evident that the periodic focussing and defocussing may alsobe produced by undulating the two surfaces of a record carrier which isto be read through the record carrier body.

Instead of the sinusoidal variation of the excursions, as shown in theFigures, it is of course also possible to select a different variationof the excursions, such as a triangular variation, provided that saidvariation is periodic.

The concept underlying the invention is not restricted to a rounddisk-shaped record carrier, but may be also used for other recordcarriers such as a record carrier in the form of a tape.

Furthermore, the concept underlying the invention is not restricted to aspecific type of information. Instead of a television programm it isalso possible to store other data on the record carrier, such asphotographic data or digital information.

The present invention is also applicable to a previously proposed recordcarrier U.S. Ser. No. 442,396 filed Feb. 14, 1974 , in which the tracks,viewed in the lateral direction of the tracks, exhibit periodicexcursions, whose period is substantially greater (for example 1000times) than the average period of the areas in the tracks, while theamplitude of the excursions is smaller (for example five times) than thewidth of the tracks. Such a record carrier with "oscillating" tracks maybe read with the aid of only one radiation beam and one detector. Theinformation can be derived from the high-frequency component of thedetector signal, whilst the low-frequency component provides anindication of the magnitude and the direction of a possible misalignmentof the read spot relative to the track to be read.

If, according to the invention, a record carrier is provided with trackshaving periodic excursions in the plane of the tracks and with a surfacewhich has periodic excursions, a small number of optical elementssuffices both for reading the information and for obtaining controlsignals for focussing the read beam and centering the read spot. Use maythen be made of the apparatus of FIG. 2. The electrical signal suppliedby the detector contains both the information and indications about thedegrees of misalignment of the read spot and the focussing of the readbeam.

If only one detector is used for reading the record carrier withoscillating tracks and an undulating surface, care must be taken thatthe spatial frequencies of the oscillations and of the excursions of thesurface differ sufficiently, so as to enable a correct distinctionbetween the signals for focussing control and those for alignment in theelectronic circuit. The frequencies of the electrical control signalsshould have a spacing relative to each other which corresponds to forexample the control band width in the electronic circuit 10.Furthermore, care must be taken that the spatial frequencies of theoscillations of the tracks and of the excursions of the surface have noharmonics in common.

The method of detecting the focussing of the read beam described abovemay only be used when the deviation of the distance between the plane ofthe information structure and the objective lens (18) is not excessive.According to the invention, however, a focussing coarse control can beachieved by a suitable choice of the size of the detector and bymeasuring the d.c. component of the detector signal. The d.c. componentof the detector signal exhibits a substantial and virtually linearvariation within a certain range (for example 10 to 40 μm) of deviationsbetween the actual and the desired plane of focussing, if theradiation-sensitive surface of the detector has certain dimensions (forexample 0.4 mm by 0.4 mm). By measuring the d.c. component in theelectronic circuit and comparing it with a reference value coarsecontrol can be achieved. Such a coarse control is, for example,necessary at the beginning of the read operation, upon insertion of therecord carrier in the read apparatus.

What is claimed is:
 1. A record carrier on which information is storedin an optically readable structure of areas and intermediate regionsarranged along elongated tracks, said structure being readable with aservo focussed read beam of radiation, said areas having a differentinfluence on said read beam of radiation than the intermediate regions,said areas having a length dimension in a direction substantiallyparallel to said tracks and a width direction substantially parallel toa major surface of said carrier, the improvement wherein at least one ofsaid major surfaces of the record carrier exhibits periodic excursionsof fixed amplitude and frequency in a direction perpendicular to saidlength and width dimensions of said areas for introducing dither intosaid servo focussed read beam, the period and amplitude of saidexcursions being substantially greater than the average period of theareas in the tracks.
 2. A record carrier as claimed in claim 1, which isdisk-shaped, the optically readable structure being aradiation-reflecting structure, the carrier body beingradiaton-transmitting, and the surface of the carrier body which facesaway from the optical structure constituting both the plane of incidenceand the plane of emergence of the read radiation, wherein the surface ofthe record carrier body on which the radiation-reflecting structure isdisposed exhibits said periodic excursions.
 3. A record carrier asclaimed in claim 1, in the form of a thin disk or foil, the opticallyreadable structure being a radiation reflecting-structure, the carrierbody being radiation-transmitting, and the surface of the record carrierbody which faces away from the optical structure constituting both theplane of incidence and the plane of emergence of the read radiation,wherein the surface of the record carrier body which faces away from theoptical structure exhibits said periodic excursions.